TM 5-852-4/AFM 88-19, Chap. 4
been measured as high as 70F in the summer.
5-852-4/AFM 88-19, Chap. 4 even though shallow, wide
Because of its high specific heat, even relatively cold
ditches are more susceptible to icing. A 10 percent
water may have significant thawing capacity as
transverse slope should be used on the bottom of the
demonstrated by the use of cold water to pre-thaw
ditch. When cutting into permafrost containing ground
gravels and to remove frozen silt by sluicing in gold
ice cannot be avoided and natural stabilization will not
mining operations. In the warmer permafrost areas thaw
occur, cannot be relied on, or would involve
zones readily develop under surface drainage channels;
unacceptable settlements and/or erosion, it may be
even temporary wastage of water on the surface during
possible to over-excavate the ditch and backfill to the
construction may produce thaw zones 10 or 20 feet deep
desired cross-section with non-frost susceptible material
which may remain unfrozen for decades thereafter.
of sufficient thickness to prevent summer thaw from
Wells drilled through permafrost should not be allowed to
reaching the underlying ice. On the other hand, it may
discharge indiscriminately on the ground surface in
be found that ditching at the site is simply impractical.
permafrost areas. Water absorbs solar radiation much
An alternative then is to place the basic facilities on fill so
more effectively than soil. Therefore, care must be taken
that all need for cutting into permafrost is avoided.
d. Ditches in permafrost areas should be as
in permafrost areas to slope surfaces at and near
facilities so that surface water from snow melt or rainfall
short as possible.
A reasonable slope is 0.003.
is drained away and ponding is positively prevented.
Minimum width should be about 2 feet at the bottom with
Wastewater from buildings, particularly hot water such as
the actual width made sufficient to handle spring run-off.
waste steam condensate, must never be allowed to
Side slopes of I on 2 are usually suitable. Where
discharge on or into the ground near a permafrost
seepage into the ditch from side slopes will cause
foundation, even in small amounts. Good surface
erosion and sloughing, where sloughing will occur as a
drainage is also important in seasonal frost areas to
result of thaw-weakening in spring and summer, or
minimize frost action. In permafrost areas, natural
where control of permafrost degradation is required,
subsurface seepage patterns in the annual thaw zone
blankets of free granular, non-frost-susceptible material
should be considered during site selection to avoid
may be employed on the slopes as discussed in
problem locations. However, it may also be possible to
paragraph 4-19.
e. Subsurface drainage systems, including
modify or control subsurface flow by judicious use of
techniques for locally raising the permafrost table at
trench drains, are not usually practical in areas of deep
critical locations, such as by placement of fill or use of
seasonal frost or permafrost unless they can be placed
shading or reflective surface color. One of the benefits
in ground which will be unfrozen at the time they need to
sought from painting the runway white at Thule,
function.
f.
Greenland, was the diversion of summer seepage flow in
If cellars or basements are attempted under
the annual thaw zone by raising of the permafrost table
heated buildings in permafrost containing ground ice, the
105
under the pavement to act as a dam .
gradual melting of ice under and around the warm cellar
b. Steam, water, and sewer lines must be kept
will cause settlement not only of the building foundations
completely tight. At an Alaskan facility minor leakage
but also of the surrounding ground. The results will be
from an overhead steam line and resultant slow drip of
development of a dish-shaped depression surrounding
condensate at the edge of the foundation contributed to
and under the facility and an increasingly difficult water
thaw of permafrost to about 18 feet over a relatively short
control problem in the cellar, which becomes in effect a
period.
sump for both surface run-off and permafrost melt water.
c. Drainage ditches cut into ground underlain
An ordinary drain trenched from the cellar to a low point
by permafrost containing ground ice or into permafrost
would soon freeze. Therefore, an endless problem of
itself should be avoided if at all possible because of the
pumping and disposal of seepage water may be
settlement and ground instability problems which will
presented to which there is no good solution.
g. Drainage from flat roofs has often been
result from thawing. Thawing of ice wedges may lead
surface drainage in unplanned directions. Undercutting
piped down through the interior of buildings and into dry
and sloughing of drainage ditch slopes may cause silting
wells outside the foundation. Such systems have a
and other problems. In soils or rocks capable of
history of problems in winter. The dry well and pipe
bridging, sing-holes and underground drainage channels
drainage system outside the building commonly freezes
may develop which may endanger even somewhat
up, and water backs up within the pipe inside the
distant foundations. Under some conditions, it may be
building. In summer, however, the contrary problem may
advisable to allow natural stabilization of the drainage
exist of some local thawing of permafrost near the dry
effects to occur. This stabilization will occur most easily
well from discharge of relatively warm water, heated on
if ditches can be made shallow, penetrating only part of
the roof. No good solutions to this roof
the annual thaw zone, rather than narrow and deep, TM
4-163
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